Rigid-Body Dynamics

Below are selected topics from rigid-body dynamics, a subtopic
of classical mechanics involving the use of Newton's laws of
motion to solve for the motion of rigid bodies moving in 1D,
2D, or 3D space.B.11 We may think
of a rigid body as a distributed mass, that is, a mass that has
length, area, and/or volume rather than occupying only a single point
in space. Rigid body models have application in stiff strings
(modeling them as disks of mass interconnect by ideal springs), rigid
bridges, resonator braces, and so on.

We have already used Newton's
to formulate mathematical dynamic
models for the ideal point-mass (§B.1.1), spring
(§B.1.3), and a simple mass-spring system (§B.1.4).
Since many physical systems can be modeled as assemblies of masses and
(normally damped) springs, we are pretty far along already. However,
when the springs interconnecting our point-masses are very stiff, we
may approximate them as rigid to simplify our simulations. Thus,
rigid bodies can be considered mass-spring systems in which the
springs are so stiff that they can be treated as rigid massless rods
(infinite spring-constants
, in the notation of §B.1.3).

So, what is new about distributed masses, as opposed to the
point-masses considered previously? As we will see, the main new
ingredient is rotational dynamics. The total momentum
of a rigid body (distributed mass) moving through space will be
described as the sum of the linear momentum of its
center of mass (§B.4.1 below) plus the angular
momentum about its center of mass (§B.4.13 below).